Foundations within RPA

[Gary Prentice]

I think the 20% incursion went with the earlier BS5837.

 

Now I understand the extent would be down to the arboriculturist, basing the decision on the trees condition, the likely rooting zone and maybe some trial digs to map the existing root zone

[Dave177]

I seem to remember reading that in exceptional circumstances a healthy tree could survive up to 50% root loss. However this is maximum and in reality you wouldnt want to go anywhere near that. I would suggest if the design of the building can be changed or the footprint moved then do this. Otherwise, dig some trial plots to find where the tree is rooting and the you can decide on a way forward. You will be suprised at the lack of roots in some cases

[oslac]

I've seen designs where the water from the roof has been directed under the slab to address moisture loss.

 

As for research, I'm not aware of any.

 

We have been creating methodology for some time to allow structures to be constructed over RPA's. It started with garages and now it mostly involves house.

 

Generally, it is easy to deal with the RPA, the big problem though is the crowns and many developers haven't got the hang of crown growth and future crown growth.

 

Micro-piles with an above ground ring beam. Concrete floor cast over a clay former which disintegrates. If required, channel the roof water under the building. There are other site-specific add-ons which is likely to be required such as de-compaction prior to starting works.

 

I would say that it is easier to demonstrate that the RPA will not be affected by the construction of a house than say a parking bay or terrace. I have never recommended geocell of any sort within this methodology, perhaps someone could explain the point of it in this instance.

 

Someone mentioned about the loss of RPA and how much a tree can withstand. I would ask the question which is, what is an RPA? Yes I know what the BS says but what is the RPA? How did the 12 times rule appear because from my knowledge, many tree roots extend well beyond this distance and so some bright spark has made a judgement call between how much root removal a tree can cope with balanced against the need to develop land in close proximity to trees. I also know the answer because I asked the BS author during his seminar travels how the 12 times rule occurred and he admitted that it was a judgement call based on no research . . and we all fall into line.

[Chris at eden]

Dave, I'm not sure if I'm thinking along similar lines to Gary but I'm wondering WHY piling may be more expensive.

 

It's like a conversation I'd have had with a builder we've used before (but no more) "Don't tell me it's more expensive, tell me WHY it's more expensive...."

 

Labour, machinery, materiels for traditional = X

Labour, machinery, materiels for piling = Y

 

The difference between the 2 is?

 

From an inexperienced, non-technical perspective it might be assumed that piling could be quicker, more efficient and cheaper or does it suffer from the 'because it's non-standard it comes at a premium' or are there tangible reasons why it could be more costly?

 

Piles for a housedeck are installed by hand with an auger. Strip footings outside of the RPA are done with a mini digger. Its a none standard construction also and has to be designed by an engineer.

[Chris at eden]

I seem to remember reading that in exceptional circumstances a healthy tree could survive up to 50% root loss. However this is maximum and in reality you wouldnt want to go anywhere near that. I would suggest if the design of the building can be changed or the footprint moved then do this. Otherwise, dig some trial plots to find where the tree is rooting and the you can decide on a way forward. You will be suprised at the lack of roots in some cases

 

Tree roots extend up to 3 times the tree height apparently, I forget the source. Based on this the RPA probably is 50% loss of the finer roots but importantly it retains the larger structural roots.

[oslac]

Tree roots extend up to 3 times the tree height apparently, I forget the source. Based on this the RPA probably is 50% loss of the finer roots but importantly it retains the larger structural roots.

 

Why is the retention of the structural roots (other than anchorage) be more important than the loss of the fibrous root system and associated root hairs? If water and dissolved nutrients are unable to enter into the tree's system, then the tree will stress and possible die.

[Chris at eden]

Why is the retention of the structural roots (other than anchorage) be more important than the loss of the fibrous root system and associated root hairs? If water and dissolved nutrients are unable to enter into the tree's system, then the tree will stress and possible die.

 

Think you just answered your own question there mate. Anchorage. If the tree falls over it has no potential to recover. Plus you will have fibrous roots on the inner RPA anyway and I would imagine its easier for a tree to regenerate small roots than much larger ones.

 

Not saying fibrous roots are not important (so I probably worded it poorly) as they clearly are but, would you rather recommend removing 50% of the fine roots or 50% of the structural roots? What do you think would be more damaging? To be fair the removal of 50% of the structural roots would automatically remove way over 50% of the fibrous but I'm going off piste now. My point was that if you trenched around the RPA that would probably remove 50% of the fine roots based on roots fanning out and becoming fine in that area. I have no source for this its just me speculating to respond to the other post.

 

Other opinions may differ.

 

Cheers

[Chris at eden]

I think the 20% incursion went with the earlier BS5837.

 

Now I understand the extent would be down to the arboriculturist, basing the decision on the trees condition, the likely rooting zone and maybe some trial digs to map the existing root zone

 

 

20% offset went with the 2005 standard but the 20% incursion into the RPA for hard surfacing still exists.

[oslac]

Think you just answered your own question there mate. Anchorage. If the tree falls over it has no potential to recover. Plus you will have fibrous roots on the inner RPA anyway and I would imagine its easier for a tree to regenerate small roots than much larger ones.

 

Not saying fibrous roots are not important (so I probably worded it poorly) as they clearly are but, would you rather recommend removing 50% of the fine roots or 50% of the structural roots? What do you think would be more damaging? To be fair the removal of 50% of the structural roots would automatically remove way over 50% of the fibrous but I'm going off piste now. My point was that if you trenched around the RPA that would probably remove 50% of the fine roots based on roots fanning out and becoming fine in that area. I have no source for this its just me speculating to respond to the other post.

 

 

 

Other opinions may differ.

 

Cheers

 

Mmmm. My post wasn't really about one set of roots versus another. I believe that nature doesn't do anything for fun and as such all roots are needed. The point is, if you remove all of the roots outside of the RPA which could be a considerably proportion of the whole amount of roots, how come that is OK in planning terms and OK in BS terms and OK in Tree Officer and Consultants terms. So whats the point of these roots outside of the RPA and where is the evidence that 12 times the trunk diameter is the magical number between tree being happy (Planning, BS etc) and unhappy (Tree Officer etc). Why not 15 times diameter or 11 times diameter.

[Gary Prentice]

10x would be a lot easier to calculate:biggrin: